1. Field of the Invention
This invention relates to a constant voltage source circuit to be used in an audio system or the like.
2. Description of the related Art
Many of devices operate on a supply of a constant voltage, for example, an audio system provided in an automobile and supplied with power by a car battery is typical of such devices. In this kind of device, the constant voltage source circuit supplying the electric power thereto must operate stably at an input voltage maintained at a predetermined voltage level. Sometimes, however, the voltage input to the constant voltage source circuit will fall below the predetermined level when for example, the voltage of the car battery is lowered but nevertheless the operating conditions of the audio system connected to the car battery must be kept stable.
Namely, the characteristics thereof such as ripple rejection or the like, must not be affected even when the output voltage thereof is lowered in accordance with the lowering of the voltage input to the constant voltage source circuit.
FIG. 5 shows an example of a conventional constant voltage source circuit.
In the conventional constant voltage source circuit shown in FIG. 5, when an input voltage V.sub.IN is higher than a predetermined voltage level V.sub.IN(S), i.e., when the constant voltage source circuit is in a stable condition, the constant voltage source circuit supplies a constant voltage in such a manner that both a voltage obtained by dividing the output voltage V.sub.O with resistors R.sub.1 and R.sub.2 and a reference voltage V.sub.REF are input to a differential amplifier A, and the output of the differential amplifier A is fed back to an output transistor Q.sub.1.
In the conventional constant voltage source circuit shown in FIG. 5, however, when the input voltage V.sub.IN falls below the predetermined voltage level V.sub.IN(S), i.e., when the constant voltage source circuit is no in a stable condition, the circuit does not include a means for overcoming the problems caused thereby and therefore, an output voltage V.sub.O which is nearly the same as the input voltage V.sub.IN is output therefrom, as shown in FIG. 6.
A further problem arises in that when the constant voltage source circuit is not in a stable condition, the output transistor Q.sub.1 is saturated and thus the ripple rejection characteristic is adversely affected.
FIG. 7 shows an example in which the conventional constant voltage source circuit shown in FIG. 5 is applied to a conventional audio system.
In this example, when the input voltage V.sub.IN is lowered and the operation of the constant voltage source circuit is not in a stable condition, the ripple component will appear in the voltage (V.sub.O) output by the constant voltage source circuit.
Further, the ripple rejection of a small signal amplifier As connected to the output of the constant voltage source circuit is also adversely affected by the lowering of the input voltage, and thus a problem arises in that the input voltage is oscillated while input to a power amplifier through the small signal amplifier As.
Therefore, when the input voltage V.sub.IN is lowered and the operation of the constant voltage source circuit is not in a stable condition, the above problems are conventionally overcome by immediately turning OFF the constant voltage source circuit.
But, when the constant voltage source circuit is used in an audio system, this interrupts the broadcast sound and is irritating to the listener.
FIG. 8 shows another example of the conventional constant voltage source circuit.
As shown in the figure, when this circuit operates in such an unstabilized area, the ripple components accumulated in the input voltage V.sub.IN, are eliminated by using a ripple filter composes of a resistor R.sub.8 and a condenser C.sub.2.
Accordingly, in this example, the ripple rejection characteristic is improved but, since this circuit includes a Zener diode ZD and does not have a feedback system, it is difficult to maintain the performance of this circuit at a predetermined level when in a stable condition, due to the characteristic variation of the Zener diode ZD.
The problem to be overcome is that when the constant voltage source circuit has a construction such that a large stress is imposed on the operating characteristics of the circuit when the circuit is in a stable condition, the ripple rejection will be adversely affected when the operating condition thereof is not in a stable condition. Conversely, when the constant voltage source circuit has a circuit construction such that a large stress is imposed on the ripple rejection thereof when the circuit is not in a stable condition, the operating characteristics of the constant voltage circuit when in the stable condition will be lowered.
To overcome the drawbacks mentioned above, several methods have been proposed in for example, Japanese Unexamined Pat. Publications No. 58-154019, No. 62-114014, No. 62-22125 and No. 62-295126.
Each of these publications, discloses a constant voltage source circuit in which a transfer of noise in the input voltage to the output voltage is prevented by avoiding a saturation of an output transistor by controlling that the base voltage of the output transistor when the output voltage falls below a predetermined level, by monitoring the voltage output by the circuit.
In each of these publications, the control is effected by detecting the voltage output by the output terminal of the circuit, and accordingly, many IC circuits usually must be provided downstream of the output terminal of the circuit.
Therefore, when a large load is applied to the output terminal, a long time is required to stabilize the output voltage at the rise time thereof i.e., the rise time of the output voltage is prolonged.
Further in these prior arts, since the control of the output transistor is effected by detecting this prolonged rise time of the output voltage, the circuit is apt to define this as a condition in which the output transistor is approaching saturation, and thus reduce the output by the output transistor to prevent this saturation.